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2.2.1 SSTP Packet

The following diagram shows the format of this packet when is sent on the HTTPS connection.

The fields of the header MUST be transmitted in byte order from left to right.


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Version

Reserved

C

LengthPacket

Data (variable)

...

Version (1 byte): An 8-bit (1-byte) field that is used to communicate and negotiate the version of SSTP that is used. The upper 4 bits are the MAJOR version, which MUST be 0x1, and the lower 4 bits are the MINOR version, which MUST be set to 0x0. This means that the 8-bit value of the Version field MUST be 0x10 and corresponds to Version 1.0.

Reserved (7 bits): This 7-bit field is reserved for future use. MUST be set to zero when sent and MUST be ignored on receipt.

C (1 bit): A 1-bit field that is used to indicate whether the packet is an SSTP control packet or an SSTP data packet (the data packet is used for sending a higher-layer payload). The value is 1 if it is a control packet and zero if it is a data packet.

Name

Value

Data packet

0

Control packet

1

LengthPacket (2 bytes): A 16-bit unsigned integer in network byte order that packs data for two fields, configured in the following format.


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R

Length

R (4 bits): This 4-bit field is reserved for future use. MUST be set to zero when sent and MUST be ignored on receipt.

Length (12 bits): A 12-bit unsigned integer in network byte order that MUST specify the length, in bytes, of this entire packet, including the 4-byte SSTP header (that is, Version, 7-bit Reserved field, 1-bit C field, 4-bit R field, and 12-bit Length field).

Data (variable): A variable-length field. The length of this field is equal to the value of the field Length, minus 4. This field contains either the SSTP control message when field C is equal to 1 (see section 2.2.2), or the payload from a higher-layer protocol when field C is equal to zero (see section 2.2.3). SSTP data packets carry PPP frames as payload (which includes PPP control frames as well as PPP data frames). For more information, see [RFC1661].

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